This invention relates to an electro-optical device, particularly for filtering, wavelength-selective switching, wavelength-selective modulation, temporal pulse shaping, and controllable dispersion
Filters and other wavelength selective elements are widely used as passive devices relying, for example, on thin-film interference or on fixed diffraction gratings.
One kind of active device, such as a tunable or electrically-controllable filter, relies on interference. These include two-beam interferometers (e.g., Michelson, Mach-Zehnder) and multiple-beam interferometers (e.g., Fabry-Perot), where the optical path length can be varied by various mechanisms, e.g., by using thermal, piezoelectric or electro-optic effects. Filters utilizing these interferometers are usually sensitive to environmental perturbations, since variations of a fraction of the wavelength in the optical path length may substantially alter the filter performance.
Other types of tunable or electrically-controllable filters rely on diffraction, e.g., Bragg diffraction. In this type of filters, extremely wide free-spectral range, as well as good spectral resolution, can be attained, and these filters are usually less sensitive to environmental perturbations than the interference-based filters. The most widely used filters of this kind are acousto-optic tunable filters, disclosed, for example in the following publication: xe2x80x9cAcousto optic tunable filterxe2x80x9d, S. E. Harris and R. W. Wallace, Journal of the Optical Society of America 59, 744 (1968). However, the acousto-optic tunable filters require continuous injection of an acoustic wave into the material, and therefore consume relatively high electrical power. In addition, they shift the frequency of light, and are inherently slow (typical switching speed exceeding 500 nsec), which makes them impractical for packet switching in high-speed optical communication systems.
Some of the above problems can be solved by using an electro-optic tunable filter, where an electro-optic grating is formed by placing inter-digital electrodes on the surface of an electro-optic material. In this case, the electric field has a suitable shape for filtering in the material only in a small region close to the electrodes. This device is mainly used with optical waveguides, but not with freely propagating beams in bulk crystals. Electro-optic modulators in bulk could be used only with relatively wide period of the electrodes, e.g., 300 xcexcm as disclosed in the following publication: xe2x80x9cDigital electro-optic grating deflector and modulatorxe2x80x9d, J. M. Hammer, Applied Physics Letters 18, 147-149 (1971). In this case, high spectral resolution cannot be achieved.
In the last few years, techniques of patterning the domain structure of ferroelectric crystals such as LiNbO3, KTiOPO4 and LiTaO3 have been developed, and disclosed for example in the following publications: xe2x80x9cQuasi-phase-matched optical parametric oscillator in bulk periodically poled LiNbO3xe2x80x9d, L. E. Myers et al., J. Opt. Soc. Am B 2102-2116 (1995), and U.S. Pat. Nos. 5,193,023 and 6,074,594. According to these techniques, a permanent grating of the non-linear susceptibility can be written into the material, for example, by applying once a very high field through patterned electrodes. The grating period can be in the range of several microns. The ferroelectric materials that can be domain-patterned are commercially available in large quantities, and the method of patterning the domains is well established.
The reversal of the ferroelectic domains also reverses the sign of the electro-optic coefficient. This is disclosed, for example in the following publication: xe2x80x9cElectric-field induced cylindrical lens, switching and deflection devices composed of the inverted domains in LiNbO3 crystalsxe2x80x9d, M. Yamada, M. Saitoh and H. Ooki, Applied Physics Letters 69, 3659-3661 (1996). Hence, applying an external electric field will create a refractive index pattern inside the material, since the same electric field that increases the refractive index in one domain will reduce the refractive index in the inverted domain. Hence, the refractive index pattern depends also on the domain pattern of the ferroelectric crystal, and not only on the external electric field. Electro-optic devices based on domain-patterned ferroelectric crystals were used for deflection, modulation and beam focussing, as disclosed, for example, in U.S. Pat. No. 5,786,926.
There is accordingly a need in the art to facilitate the tuning and controlling of the propagation of an optical signal by providing a novel electro-optical device and method of the kind utilizing the electro-optical effect in domain-patterned ferroelectric crystals.
The device of the present invention can operate as an electronically-controlled optical filter and other wavelength selective optical elements, such as a switch, modulator, dispersion compensator and polarization mode dispersion compensator, having wide free spectral range, high spectral resolution, low power consumption and fast switching speed. Optical filters and other wavelength-selective optical elements according to the invention can be used with freely propagating beams. Generally speaking, the device according to the present invention enables wavelength selection, and utilizes a method for affecting the phase of light impinging on the device in a wavelength-dependent manner.
Electrically-controlled optical filters and other wavelength-selective optical elements are very important for optical communication systems. Dense wavelength division multiplexed (DWDM) optical communication systems require devices that can selectively manipulate different, closely spaced optical channels. One example of such an optical device that is often used in wavelength-routed systems is an add/drop multiplexer (ADM), where data on one or more optical channels are dropped at a certain node, while new data is added on the same wavelengths.
The conventional ADMs are mainly passive devices. It is desired to make an ADM as an active (or re-configurable) device that enables to dynamically change the dropped and added channels, in order to adjust the number of channels to the varying traffic volume in different locations in the system. More advanced systems should be able to switch data packets at predetermined wavelengths, and require a switching time in the nanosecond range.
The optical device according to the invention relies on a structure made of a ferroelectric crystal such as LiNbO3, KTiOPO4, LiTaO3, MgOLiNbO3, KTiOAsO4, RbTiOAsO4, KNbO3, with patterned domains, and electrodes on the surface of the structure. In these domain-patterned crystals and in contrary to the standard mono-domain crystals, the refractive index pattern in the material is determined not only by the electrodes on the surface, but by the domain pattern as well. For a given domain pattern in the structure, by directing a light component of a predetermined wavelength onto the structure at a certain angle of incidence satisfying the Bragg condition for this predetermined wavelength, and applying voltage to the electrodes, this light component can be appropriately deflected to propagate with a certain direction. This can be used for selecting the light component of the predetermined wavelength from input light containing a plurality of wavelengths including this predetermined wavelength.
There is thus provided according to one aspect of the present invention, a method affecting the propagation of input light, the method comprising the steps of.
(i) passing the input light through an electro-optical device composed of at least one domain-patterned ferroelectric crystal structure with inverted domains, and an electrodes"" arrangement on the surface of the at least one structure connected to a voltage source; and
(ii) applying voltage to the electrodes, thereby causing at least one of the following: deflecting a light component of a predetermined wavelengths contained in the input light from the direction of propagation of the input light so as to direct said light component in a reflection output direction, while allowing the other light components to propagate in said direction of propagation of the input light; changing the phase of the input light in a wavelength-dependent manner; and separately controlling the wavelength-dependent transmission and dispersion for light components of different polarizations contained in the input light.
According to another aspect of the present invention, there is provided a method of selecting a light component of a predetermined wavelength from input light formed of a plurality of light components of different wavelengths including said predetermined wavelength, the method comprising the steps of:
directing the input light onto an electro-optical device at an angle satisfying the Bragg condition for said predetermined wavelength, wherein said electro-optical device is composed of a domain-patterned ferroelectric crystal structure with inverted domains, and an electrodes"" arrangement on the surface of said structure connected to a voltage source; and
applying voltage to the electrodes, thereby deflecting the light component of said predetermined wavelengths from the direction of propagation of the input light direction to propagate in a reflection output direction, while allowing the other light components to propagate in said direction of propagation of the input light.
According to yet another aspect of the present invention, there is provided an electro-optical device comprising:
(a) a domain-patterned ferroelectric crystal structure with inverted domains;
(b) an electrodes"" arrangement on the surface of said structure connectable to a voltage source to selectively apply voltage to each of the electrodes;
(c) at least one input port for directing at least one input beam onto the structure; and
(d) a control unit operable for selectively affecting at least one of the following: an angle of incidence of the input beam onto the structure, the shape of the input beam, propagation of light components of different polarizations contained in the input beam, and a voltage applied to the electrodes.
The device can operate as a controllable filter, the filter transmission being controllable by varying at least one of the following: structure of the inverted domains, shape of the electrodes, voltage applied to each of the electrodes, the shape of the input beam, and an angle of incidence of the input beam.
The device can operate as a wavelength selective switch. The wavelength selection can be performed by directing the input beams onto said structure at different angles of incidence, thereby providing output beams ensuing from the structure with different angles of propagation, the angle of propagation of the output beam depending on the wavelength of the input beam. The device can thus operate as an electrically-controlled add, drop or add/drop switch. The device can also operate as a wavelength selective modulator.
By using the device according to the invention in an optical communication system, for example of the kind utilizing an optical amplifier with wavelength dependent gain, equalizing of the amplified power as a function of wavelength can be achieved. The device can be used for voltage controlled pulse shaping. In particular, this can be used to correct the dispersion accumulated in optical or fiber-optic communication system, and can handle changing levels of dispersion by appropriate selection of patterned domain structure and applied voltage through the electrodes.
The device can be used for wavelength dependent packet-switching in optical communication systems. By providing polarization beam splitters and retarders in the input port and at an output of the device, the wavelength-dependent transmission and dispersion can be separately controlled for different polarizations of light. For example, it can be used to correct polarization dependent loss and polarization mode dispersion in optical systems.